Patent GB-536465-A (GB)
Title: Light control by polarisation
IPC: G02B27/28
Filed: 1939-12-29
Abstract: 536,465. Lighting control by polarization photographic projection-printing apparatus. TECHNICOLOR MOTION PICTURE CORPORATION. Dec. 29, 1939, No. 33034. Convention date, March 13. [Classes 97 (i) and 98 (i)] The intensity of a light beam, e.g. in a photographic or kinematograph film printer is continuously controlled, while diverting unwanted energy from the axis of the beam, by passing the beam through a system comprising a polarizer and an analyser fixed relatively to each other and between which is an angularlyadjustable half-wave plate. The arrangement shown in Fig. 1 comprises two calcite prisms C<SP>1</SP>, C<SP>2</SP>, each comprising two cemented components P', <SP>p2</SP>, P<SP>21</SP> P<SP>22</SP> whose polarizing axes are directed as indicated by the hatching.. Between the prisms is placed a half-wave plate q angularly adjustable in its own plane. An unpolarized light ray L is divided by prism C<SP>1</SP> into an ordinary ray L<SP>1</SP> and extraordinary ray L2. With the optical axis of plate q directed as shown at I, the planes of polarization of rays L<SP>1</SP> and L<SP>2</SP> are not changed ; ray L<SP>1</SP>. is extraordinary for prism C<SP>2</SP> and is deviated along L<SP>12</SP>, L<SP>2</SP> is ordinary and passes straight on as shown at L<SP>21</SP>, while rays along L<SP>11</SP> and L<SP>22</SP> are extinguished. If the optical axis of plate q is rotated 45 degrees to II, the planes of polarization of rays L<SP>1</SP> and L<SP>2</SP> will be rotated 90 degrees so that L<SP>1</SP> is ordinary for prism C<SP>2</SP> and goes straight on along L<SP>11</SP>, L<SP>2</SP> is extraordinary and is deviated along L<SP>22</SP>, while rays along L<SP>12</SP> and L<SP>21</SP> are extinguished. Thus areas I<SP>12</SP> and I<SP>21</SP> of a screen M will be illuminated with about 50 per cent. intensity with the axis of plate q in position I and areas I<SP>11</SP> and I<SP>22</SP> will be dark. With the axis of plate q in position II these conditions are reversed, so that as plate q is rotated, the intensity of illumination of each area varies continuously from about 50 per cent. to zero, or the reverse. While any or all of these areas may be used, in the application to photographic projection printing apparatus described, only one area I<SP>11</SP> is used, the beams illuminating the remaining areas being intercepted, e.g. by blackened surfaces. Fig. 4 shows the arrangement schematically. Light from a source 10 with reflector 11 passes through a condenser 12, baffle 17, and water cell 16 to the polarizing device, which may be of the form shown in Fig. 1 with collimating lenses 21, 22 on the faces b, c, of the prisms C<SP>1</SP>, C<SP>2</SP>. Thence it passes via lens 23 to the negative aperture 24 from which the image is projected by an objective 25 to the positive aperture 26. Instead of the composite prisms C<SP>1</SP>, C<SP>2</SP>, Wollaston or Rochon prisms, or totally reflecting calcite prisms such as that shown in Fig. 3a with their polarizing axes at right angles, may be used, and Fig. 5 show a mounting using prisms of the last mentioned kind. The casing 40, which supports the other elements (not shown) of the projection printing system houses the water cell 16, and the prism holder 42 in which the prisms D<SP>1</SP>, D <SP>2</SP> are secured on the one hand by screwed rings engaging the collimating lenses 21,. 22, and on the other by spring-pressed blocks 47,. 48. The half-wave plate holder is mounted in ball bearings 57 and has a toothed periphery engaging a rack 59 whereby it may be rotated, the rack being operable by means for controlling the intensity of the light beam, e.g. from the negative film to be printed as described in Specification 536,464. Backlash between the rack and the toothed plate mount is prevented by a spring-pressed roller 60.